Carburetor SynchronizationWith the proliferation of the Rotax 912

80 hp and the Rotax 912S 100 hp engines, the topic of carburetor synchronization has come to the forefront. Until about the 1980s, the popularity of Continental and Lycoming engines dominated the general aviation market, these engines used a single carburetor providing for a single source of air and fuel to the cylinders. The use of dual carburetors was primarily relegated to the area of the two-stroke ultralight market. And, even with these engines, the process of carburetor synchronization was quite simple and reliable. How-ever, with the popularity of the Rotax 9 series engines, it has become import-ant to under-stand a little bit more about how the induc-tion system works on this amazing little powerhouse. This understand-ing is important not only from a maintenance standpoint, but from a pilot’s per-spective as well.

The Rotax 912 is essentially two engines connected to a single crankshaft and gearbox with both the left and right sides of the engine having their own independent carburetor, ignition, and exhaust sys-tem Figure: 1. As you might well imagine, having two engines trying to run a single propeller requires a bit of choreography between the right and left side of the en-gine in order to make things run smoothly. Most of us, who have spent a consider-able amount of time in the air, can remember a time when one of the cylinders on a

Figure: 1 The Rotax 912 is two separate engines connected to a common crankshaft

four-cylinder engine just quit firing, maybe from fouled spark plugs, or a plugged fuel

injector. Regardless of the source, if you have ever lost a cylinder, it likely got your attention. Now imagine losing two cylinders. This is nothing short of an all-out assault on your engine and airframe. The shaking can be so violent that the fear of the mo-tor departing the airframe becomes a realistic concern. With an engine like the Rotax 912, which has the right and left side induction systems isolated from each other, you can see the potential hazard with having one throttle wide open and the other at idle. The resulting reaction of the engine would be similar to the scenario of losing two cylinders in our previous example. In fact, we now train pilots differently in a Rotax powered aircraft by teaching them to advance the throttle to full throttle in the event of a violently shaking engine. The reason for this is that on most Rotax powered air-craft the throttles are spring-loaded to the full throttle position. As a result, in the un-likely event of a throttle cable failure, pulling the one remaining throttle cable back to idle when the engine starts to shake just exacerbates the problem. By advancing the throttle to full throttle, it allows the throttle springs to bring both carburetors to the (same) full throttle position. This allows the engine to run smoothly and the aircraft to be flown to the nearest airport where the engine can be shut off for a dead stick landing, a better scenario than losing the engine power entirely. Theoretically, at full throttle the carburetors are perfectly synchronized by the throttle arms hitting the full throttle stops simultaneously.

So we’ve identified that the Rotax 912 is basically two engines running in synchro-nicity at full throttle. Having one throttle cable adjusted in a slightly different position (let’s say 1/8” of extra cable), compared to the other throttle cable, at full throttle

Figure: 2 Clamping off the crossover (Balance) hose

would result in only a minis-cule differential in the manifold pressure of the two intake man-ifolds. However, if the throttle arms are in the idle position 1/8” difference in throttle cable length would result in a mas-sive pressure differential be-tween the two intake manifolds. And as a result, the engine would run extremely rough. At idle a very small adjustment makes a significant change in the pressure differential. And as we open the throttle wider the pressure differential between the two manifolds decreases. The most important synchroni-zation point is at idle and just off idle.

A balance tube has been de-signed into the engine shown in purple in Figure: 1. This is a tube which runs from one in-take manifold to the other. The theoretical basis for this is that if one throttle is slightly further open, and as a result has a slightly higher manifold pres-sure, the fuel/air mixture will be diverted through this crossover

tube to the other intake manifold equalizing the manifold pressure. When both throt-tles are open exactly the same amount and the manifold pressure is identical there is no flow from one side to the other through the balance tube. And when there is a sig-nificant imbalance or mis-synchronization the flow through the balance tube is sub-stantial. Understanding this has allowed us to develop a quick and simple field test to identify engines with poor synchronization. By taking a pair of hose clamp pliers and momentarily blocking off the rubber hose connecting the balance tube to the intake manifolds while the engine is running we can identify a poorly synchronized engine Figure: 2. If we block off the balance tube and the engine continues to run smoothly, there is little flow from one side to the other. However, if we block off the balance tube and the engine shakes a great deal, it is an indication that the engine is in dire need of proper synchronization. On one occasion the shaking was so bad after blocking off the crossover hose that the carburetor was shook loose from the intake manifold.

Figure: 3 Clamp Pliers

The absolute best vacuum hose pliers to use for this oper-ation, ironically can be obtained from Harbor Freight. They make a very low cost set of plastic vacuum hose pliers with a locking device. The jaws on the plastic vacuum hose pliers have a very nice rounded “V” section that fits perfectly in between the cross-over tube and the in-

take manifold Figure: 3. Simply pinch the hose between the pipe and the manifold fitting without pinching the aluminum pipe. Now, as you might imagine, this simple test is in no way a substi-tute for doing a proper carburetor synchronization.

Some of the characteristics associated with mis-synchronized carburetors include: Overall vibration causing wear and tear on the airframe and engine. Rough running at idle, too low of idle speed, including engine stopping during final approach. Exces-sive wear on the gearbox resulting in an increase in the amount of steel both in the oil filter as well as on the magnetic drain plug. Troubles with the needle and seat within the carburetor seating properly, particularly at idle. This causes excessive fuel in the float bowls and a rich mixture. The rich mixture, in turn, makes the engine run rough-er exacerbating the shaking problem.

Rotax provides a fairly comprehensive set of instructions for proper carburetor synchronization in the line maintenance manual, downloadable from the Rotax web-site (<http://flyrotax.com/>). However, the myriad of different throttle linkage systems used throughout the light sport industry require that you extrapolate on to the syn-chronization procedures and adapt them to your specific type of throttle actuation system. No matter how your linkage system is designed, the basic procedures in-volve installing a manifold pressure monitoring system into each of the two intake manifolds and then adjusting the carburetors to achieve an identical manifold pres-sure both when the throttle is at “idle” as well as just “off idle”. We use a Synchro-mate II Figure: 4. This is a digital synchronization tool built specifically for the Bing Carburetors and has the added advantage of allowing the synchronization of the tool prior to each use. By disconnecting the balance tube from the engine and attaching the synchronization tool into each manifold, we can measure the differential pressure between each intake manifold. On the Synchromate II, when the digital bars are in the center position Figure: 5, the pressure in each manifold is equal. Bar movement

Figure: 4 Synchromate II installed into manifolds

to either side will indicate the manifold with the lower pressure Figure: 6. The chal-lenge becomes understanding your throttle linkage system. The system should have a physical idle throttle stop normally at the throttle, which should be making contact simultaneously as both throttles hit the idle RPM adjustment stop, this needs to oc-cur while maintaining proper idle rpm and perfect synchronization. Now, if that isn’t enough, as you advance the throttle to move the throttle arm off of the idle RPM adjustment stop, the cable adjustment now controls the synchronization. Ironical-ly, if adjustment of the cable is necessary in order to maintain synchronization in the off idle position, it will inevitably screw up your throttle cable position in relationship to the physical idle stop at the throttle. This, in turn, will allow the throttle to be pulled against the idle stop at the carburetor. Excessive force against the idle stop can cause the light weight idle stop on the Bing carburetor to bend. This, now, causes the idle synchronization to be screwed up and as a result, the entire synchronization and adjustment process needs to be started over. If you are new to this process, it can be rather frustrating. Our recommendation is that you use an experienced LSRM (Light Sport Repairman with a Maintenance Rating) experienced with the carb synchronization process to help you through the first synchronization.

Once you become familiar with your particular airplane, have the throttle link-age set up correctly, and you understand how it works, the system is rather simple and bulletproof. Most of the problems we see related to carburetor synchronization are simply a lack of understanding about how to properly set up the linkage in re-lationship to the carburetors, poorly de-signed throttle systems, or trying to synchronize worn out carburetors that need to be rebuilt. It is a waste of time to be synchronizing the carburetors if they are not set up and working correctly. The 912 and 912S Rotax engines are amazing products. Once you become familiar with the nuances of their maintenance and operation, you can’t help but be impressed by the elegance of the design.

© Rainbow Aviation 2017 Carol and Brian CarpenterCarol and Brian Carpenter are owners of Rainbow Aviation Services in Corning,

California. For more Information visit www.rainbowaviation.com

Figure: 6 Improperly synchronized carburetors

Figure: 5 Properly synchronized carburetors